1. Field of the Invention
The invention relates to a display apparatus which is used for a television receiver, a view finder of a video camera, a light bulb of a video projector, a display of a computer, or the like and, more particularly, to an assembly of a driving circuit of such a display apparatus.
2. Related Background Art
As a recent display apparatus, attention is paid to a display apparatus having a display device such as liquid crystal device, plasma light emitting device, electrochromic device, electron emitting device, or the like in place of a CRT.
Each of those display devices has a number of pixels and has an electrode to apply an electric signal to decide an optical state of each pixel.
Further, in order to apply the electric signal to the electrode of the display device, a rigid wiring board called a bus board or a driver board and made of glass epoxy or the like and a flexible wiring board such as a TAB film or the like are used.
Such a structure has been disclosed in U.S. Pat. Nos. 5,019,201 or 5,360,943 or the like.
A case where a liquid crystal display panel is used as a display device and a film carrier having a driving IC chip is used as a flexible wiring board will now be described herein.
Hitherto, when connecting the film carrier on which a driving circuit is mounted to a display panel and a bus board, there is a drawback such that outer leads located at both ends among the outer leads on the input side and output side are likely to be damaged by a stress from the outside due to a shock, a vibration, a thermal stress, or the like.
To solve such a drawback, therefore, as shown in FIG. 6, dummy leads 8 and 9 which are electrically not related to the driving for an image display are arranged on both outsides of outer leads 6 and 5 on the input side and output side among the leads of the film carrier, thereby preventing a damage of the outer leads at both ends due to an external stress. In FIG. 6, reference numeral 1 denotes a display panel; 2 a driving circuit such as an IC chip or the like; 3 a bus board; 4 a film carrier; and 7 a soldering land for the dummy leads.
There is a case where a quality of an image which is displayed in a display area is improved by always holding the outside of the display area to display the image to either one of a white state (bright state) and a black state (dark state). Such a method is called frame driving.
FIG. 7 shows an example of a display apparatus. Four electrodes 99 for a frame driving are provided for the display panel 1 and are connected to leads for a frame driving of film carriers 4 and 4′ arranged at both ends of two bus boards 3. Reference numerals 201 and 201′ denote scanning line driving circuits of the film carriers 4 and 4′.
One of methods for the frame driving is a method whereby film carriers and driving circuits of substantially the same construction are used as all of the film carriers 4 and 4′ and all of the driving circuits 201 and 201′ and the apparatus is driven so as to supply a signal for always holding the pixels (frame pixels) on the electrodes 99 for the frame driving in a white or black state to the electrodes 99.
Another method is a method invented by the present inventors namely, a method whereby the structure of the film carriers 4′ at both ends of the bus board 3 is made different from a structure of the other film carriers 4.
FIG. 8 shows a structure of the film carrier 4′ which is used in such another method.
It is also possible to construct in a manner such that an input side and an output side of dummy leads 8′ and 9′ are electrically short-circuited and connected in order to supply a signal for, for example, always displaying in white to the frame driving electrodes 99 on the outside of a display area of the display panel 1, the input side is connected to the bus board 3, and the output side is connected to the terminal of the frame driving electrodes of the display apparatus, thereby directly supplying the signal from the bus board 3 to the display apparatus without passing through the driving circuit 201′ and driving a frame area.
In case of the above structure, the same IC chip as that of another driving circuit 201 can be used as a driving circuit 201′.
There is no need to give a special construction or special driving control for a frame driving to the IC chip.
In case of a structure shown in FIG. 6, since the dummy lead 9 electrically enters a floating state, it functions as a role of an antenna and radiation noises are generated.
To avoid such a problem, when the dummy leads 8 and 9 are short-circuited to the adjacent outer lead 5, the dummy leads 8 and 9 cannot be used for the frame driving of the display panel 1.
In the case where, for example, the dummy leads 8 and 9 are held to a ground and are fixed to a grounding voltage in order to avoid the electrical floating state, a stationary DC voltage difference occurs between the dummy leads 8 and 9 and the outer leads 5 and 6 which are adjacent thereto. At this time, particularly, in a connecting portion between the display panel 1 and the outer lead 9 on the output side, a failure such as disconnection or the like due to an electric corrosion is likely to be caused.